Infrared observation of OC-C2H2, OC-(C2H2)2 and their isotopologues

TL;DR

This study re-examines the infrared spectra of OC-C2H2 and related isotopologues, revealing new insights into their molecular structures and confirming the planarity and geometry of the trimer through experimental and computational analysis.

Contribution

It provides detailed spectral analysis of OC-C2H2 and its isotopologues, identifying a new mixed trimer and elucidating its structure using experimental spectra and ab initio calculations.

Findings

Identification of a new mixed trimer CO-(C2H2)2.

Confirmation of the trimer's planarity and structure.

Agreement of observed rotational constants with theoretical calculations.

Abstract

The fundamental band for the OC-C2H2 dimer and two combination bands involving the intermolecular bending modes nu9 and nu8 in the carbon monoxide CO stretch region are re-examined. Spectra are obtained using a pulsed supersonic slit jet expansion probed with a mode-hop free tunable infrared quantum cascade laser. Analogous bands for OC-C2D2 and the fundamental for OC-DCCH as an impurity are also observed and analysed. A much weaker band in the same spectral region is assigned to a new mixed trimer, CO-(C2H2)2. The trimer band is composed uniquely of a-type transitions, establishing that the CO monomer is nearly aligned with the a-inertial axis. The observed rotational constants agree well with ab initio calculations and a small inertial defect value indicates that the trimer is planar. The structure is a compromise between the T-shaped structure of free acetylene dimer and the linear geometry of free OC-C2H2. A similar band for the fully deuterated isotopologue CO-(C2D2)2 confirms our assignment.